1. Field of the Invention
The present invention relates to the field of optically pumped atomic clocks or magnetometers, and more particularly to atomic clocks or magnetometers that operate by exciting multi-coherent resonances using pumping of light of appropriate modulation format such as alternating polarization referred to as push-pull pumping.
2. Description of the Related Art
Conventional, gas-cell atomic clocks utilize optically pumped alkali-metal vapors. Atomic clocks are utilized in various systems that require extremely accurate frequency measurements. For example, atomic clocks are used in GPS (global positioning system) satellites and other navigation and positioning systems, as well as in cellular phone systems, radio communications, scientific experiments and military applications. A design similar to that of an atomic clock is also utilized as a magnetometer, since some of the atomic resonances are highly sensitive to the magnetic field.
In one type of atomic clock, a cell containing an active medium, such as rubidium or cesium vapor, is irradiated with both optical and microwave power. The cell contains a few droplets of alkali metal and an inert buffer gas (such as N2, any of the noble gases, or a mixture thereof) at a fraction of an atmosphere of pressure. Light from the optical source pumps the atoms of the alkali-metal vapor from a ground state to an optically excited state, from which the atoms fall back to the ground state, either by emission of fluorescent light or by quenching collisions with a buffer gas molecule such as N2. The wavelength and polarization of the light are chosen to ensure that some ground state sublevels are selectively depopulated, and other sublevels are overpopulated compared to the normal, nearly uniform distribution of atoms between the sublevels. The resonant transitions (or resonances) between these sublevels can be excited by the microwaves. It is also possible to excite the same resonances by modulating the light at the Bohr frequency of the resonance (a method currently known as coherent population trapping, or CPT), as first pointed out by Bell and Bloom, W. E. Bell, and A. L. Bloom, Phys. Rev. Lett. 6, 280 (1961), hereby incorporated by reference into this application. The changes in the population distributions of the ground state of alkali-metal atoms, introduced by the resonance, lead to a change in the transparency of the vapor, so a different amount of light passes through the vapor to a photo detector that measures the transmission of the pumping beam, or to photo detectors that measure fluorescent light scattered out of the beam. When an applied magnetic field, produced by the microwaves, oscillates with a frequency equal to one of the Bohr frequencies of the atoms, the populations of the ground-state sublevels are perturbed and the transparency of the vapor changes. If excitation by the modulated light (CPT) is used instead of the microwaves, a coherent superposition state of the ground-state sublevels is generated when the light modulation frequency or one of its harmonics matches one of the Bohr frequencies of the atoms. The changes in the transparency of the vapor are used to lock a clock or a magnetometer to the Bohr frequencies of the alkali-metal atoms.
The Bohr frequencies of a gas-cell atomic clock are the frequencies v with which the electron spin S and the nuclear spin I of an alkali-metal atom precess about each other and about an external magnetic field. For the ground state, the precession is caused by magnetic interactions. Approximate clock frequencies are v=6.835 GHz for 87Rb and v=9.193 GHz for 133CS. Conventionally, clocks have used the “0-0” resonance which is the transition between an upper energy level with azimuthal quantum number m=0 and total angular momentum quantum number F=a=I+½, and a lower energy level, also with azimuthal quantum number m=0 but with total angular momentum quantum number F=b=I−½.
Because of advances in the technology of diode lasers, there is an increasing interest in replacing the conventional atomic-resonance pumping lamps of atomic clocks with compact diode lasers. Diode lasers can be readily modulated, so it may be possible eliminate the microwave cavities and microwave field sources used to drive the 0-0 hyperfine resonance of traditional atomic clocks by using coherent population trapping (CPT) resonances, as described in H. R. Gray, R. M. Whitley, and C. R. Stroud, Opt. Lett. 3, 218 (1978), excited by diode lasers modulated at the 0-0 hyperfine frequency of the ground-state alkali-metal atom or a sub-harmonic thereof, as described in J. Vanier, M. W. Levine, D. Janssen, and M. Delaney, Phys. Rev. A 67, 065801 (2003). This type of CPT resonance has been used in atomic magnetometers, as described in S. J. Seltzer and M. V. Romalis, Appl. Phys. Lett. 85, 4804 (2004).
It has been found that the observed changes of transmitted or fluorescent light when the 0-0 resonance is excited and probed by frequency-modulated light become too small for practical use at buffer-gas pressures exceeding a few hundred torr as described in D. E. Nikonov et al., Quantum Opt. 6, 245 (1994). Broadening of the optical absorption lines degrades the CPT signals generated with frequency modulated light in much the same way, and for analogous reasons, as decreasing the Qs (quality factors) of the two tuned circuits degrades the performance of phase-shift discriminators of FM radio or television receivers. The population concentration in the end state and the suppression of the 0-0 resonance also occurs when the pumping is done with unmodulated light of fixed circular polarization, and it is independent of whether the resonances are excited by microwaves, or with the circularly polarized light that is frequency-modulated at v0/2, half the 0-0 frequency.
Conventional CPT atomic clock systems have used modulated light of fixed polarization. It has been found that much less degradation of the 0-0 CPT resonances with increasing buffer gas pressure occurs if light of fixed circular polarization is intensity-modulated at the frequency vo instead of being frequency-modulated at v0/2.
The CPT signal with pulsed light of fixed circular-polarization at very high buffer-gas pressure has about the same amplitude as the CPT signal at low pressures with frequency-modulated light. In both cases, the small signal amplitude is due to the accumulation of most of the atoms in the end state. The suppression of the 0-0 CPT signal due to optical pumping has been discussed in J. Vanier, M. W. Levine, D. Janssen, and M. Delaney, Phys. Rev. A 67, 065801(2003).
It is desirable to provide a method and system to permit the use of any alkali-metal isotope in conventional clocks, optically pumped in a conventional manner using miniature resonance lamps instead of using lasers by using multi-coherent resonances excited with multi-quantum microwave transitions.